The present invention is a method and the resulting article produced by prestressing the wall of a cylindrical structure to be used for heating or cooling a material being processed on a surface of the structure. These stresses are opposite in direction to those introduced by heat flow through the drum wall when in use. An example might be an internally heated dryer drum or the drum of a drum-and-belt type press in which the outer surface is caused to have residual compression stresses and the inner surface residual tension stresses under conditions of no heat flow through the drum.
Many manufacturing processes require the simultaneous application of both heat and mechanical pressure to a continuously moving mass of material being processed. An example might be the drying of paper. In some instances a rotating heated drum applies both heat and pressure to the moving web of paper. Miller, in U.S. Pat. No. 4,710,271, shows a hollow cylindrical drum with an internal heat source and external rolling loads as an example of this type of operation.
The situation can readily arise in which the desired combination of heating loads through the drum wall, and the superimposed external mechanical loads, produce a combination of differential expansion, thermal stress, and mechanical bending stress which exceeds the allowable design stress of the drum material. An intrinsic dilemma for a designer is that increasing thermal flux requires a thinner drum wall to reduce temperature differential across the wall. However, increasing mechanical stress requires a thicker drum wall to bear the imposed loads.
It is well known that conventional paper drying cylinders remove 5 to 7 pounds of water per hour per square foot of dryer cylinder surface. This requires a heat flow through the cylinder surface of perhaps 10,000 BTU/hr/ft.sup.2. In most cases there are only negligible mechanical forces acting on the outer circumference of the cylinder. However, a high performance paper dryer cylinder, such as that of U.S. Pat. No. 4,710,271, might require a heat flow through the cylinder wall of 100,000 BTU/hr/ft.sup.2. It could also require large external mechanical loads, such as multiple nips, with each nip exceeding a loading of 1000 pounds per linear inch. The result is a very severe combination of thermal and mechanical loading. The problem of compensating for these operating stresses has not been satisfactorily addressed prior to the present invention.
In a long hollow thin wall cylinder, with heat flow from the inside to the outside, compression is induced at the hotter surface and tension at the cooler surface. The maximum circumferential stress S (compression at the hotter side, tension at the cooler side is 1/2.DELTA.T .alpha.E/(1-.nu.) where .DELTA.T is the temperature difference from the hot to the cooler side, .alpha. is the coefficient of thermal expansion for the wall material, E is the modulus of elasticity and .nu. is Poission's ratio (Roark, R. J. and W. C. Young, Formulas for Stress and Strain, 5th ed., p 585, McGraw-Hill, New York, 1975). The stress so calculated will take the pattern of FIG. 1. Thermal flow in operation will produce a similar pattern of equal and opposite stresses (tension and compression) at the two surfaces. Thus, it is most desirable for a stress reducing mechanism to relieve the stresses at the surfaces in a manner similar to that in which they naturally occur.
Others have prestressed cylindrical objects for various purposes. However, none of these products have addressed the problems outlined above. As one example Schiel, in U.S. Pat. No. 3,946,499, preloads a cylindrical drum closed by end bells. This is done by means of a tensioned interior tie rod assembly affixed between the end bells. The stress imposed upon the wall is an axial compression load which is uniform across the wall thickness. The purpose of the preloading is to let the tie rod or rods bear the force of the internal steam pressure on the end bells instead of the cylinder wall. In concept Schiel is equivalent to a preloaded beam.
Autofrettage is the internal hydrostatic loading of a thickwalled tube, such as a cannon barrel, to create residual compression stresses at the inner surface. This produces an asymmetric stress pattern across the wall similar to that shown in FIG. 2.
Casting a metal object inherently creates residual shrinkage stresses in the casting. A cast hollow cylinder will have residual stresses due to a differential transition from the molten to the solid state. Assuming that the outer surfaces cools and solidifies before the interior mass of the wall, tension will be created in the interior portion and compression at the outer surface. The inner surface will have a smaller amount of tension or compression depending on respective cooling rates. As seen in FIG. 3, the stresses will not be equal at the two surfaces and the stress pattern does not vary evenly across the wall thickness.
Heat treating processes are frequently used, often based on the selection of specific metal alloys, to produce a certain stress pattern in a metal object. For example, Koistinen, in U.S. Pat. No. 3,216,839, teaches the creation of a residual compressive stress layer on the surface of an austenitic steel object. This is done by altering the transition of that layer to a martensitic structure. The transition form austenite to martensite causes a 3-4% increase in volume. By delaying the transition of the surface layer, upon its subsequent transition its volume increase is constrained by the already hardened bulk of the object. This constraint introduces residual compression stresses as shown in FIG. 4. Koistinen provides for altering the transition temperature of the layer either by changing its austenizing temperature, which changes its transition temperature to martensite, or by using a different alloy for the surface layer. In addition to the differences already noted, the Koistinen method requires that the quenching rate should be comparatively slow so that the smallest possible temperature gradient can be maintained between the inner core and the surface.
Japanese laid open application Sho 61[1986] 170516 teaches production of residual compression stresses in small localized areas of a pipe adjacent to disturbances such as a nipple location. A zone of tension stress surrounding the compressed area is produced incidental to production of the compressed area. At any given point the stress across the wall thickness is essentially uniform.
The above noted Japanese application rather vaguely mentions a prior art method which creates residual compression stress at the butt weld seam of a pipe. This procedure is used to avoid stress corrosion cracking in an environment where the following three essential elements all are present--a susceptible metal, tension stress, and a corrosive environment. The procedure is described in more detail in an article by Froelich et al., Nuclear Engineering International, January, 1988, pp 47-48. The process called induction heating stress improvement (IHSI) uses induction heating outside the pipe and a flow of cooling water inside the pipe to create a sufficient temperature differential across the wall so as to leave the desired residual compression stresses at the inside wall.
As will be pointed out in detail later, there are several very significant differences between IHSI and the present invention. The sole objective of IHSI is the creation of residual compression stresses adjacent the weld seam at the inside pipe wall surface. Any residual tensile stress created at the outside wall would be either incidental to the process or could actually be detrimental. Porowski et al., Nuclear Engineering International, November, 1986, pp 56-57, confirm that residual tension stresses on the outside of the pipe are undesirable. This group teaches the use of a mechanical method to induce compression at the inside wall surface. They state that thermal processes "leave high hoop residual tensile stresses in the outer half of the wall, allowing deep axial cracks and leaks".
It will be shown later that the present invention requires equal but opposite stresses at the surfaces. IHSI uses induction heating which inherently delivers heat to the interior portion of the wall as well as to the surface. King, in the Piping Handbook, 5th ed., pp 7-136 and 7-138, McGraw-Hill, New York (1967), discusses the use of induction heating for heat treatment of ferritic-steel pipe welds. He confirms that heating occurs within the wall so as to provide "more uniform temperature throughout the wall and a smaller temperature difference between the outside and inside surfaces". The context of the article is heat treatment for stress relief, not the creation of any specific stress pattern within the pipe wall. Nevertheless, application of heat in this manner would create a non-linear temperature differential across the wall thickness and would unavoidably create unbalanced stresses at the two surfaces. Depending on the particular operating conditions, the yield stress on the hot side might or might not be exceeded. In the later case, that would leave no residual tension stress at the outer surface and the stress pattern would resemble that shown in present FIG. 2. An unbalanced stress pattern of this or any other type is wholly unsatisfactory for the present invention.
For the above reasons, induction heating is generally not applicable to the present invention.
The current invention specifically addresses the problem of high thermal and mechanical loads on a cylindrical structure and provides a means for significantly reducing the contribution of thermal stress to the combined load while still permitting a high heat flux.